The act of distillation is a well-described unit operation of chemical engineering. Put in its most simplistic terms, distillation involves the vaporization of part of a liquid solution with separation and recovery of the vapor and residue of the solution. Distillation may be carried out as a batch process as shown in FIG. 1. Specifically, feed liquid is contained within a still pot 1 heated by energy source 7 causing the vapor to pass through line 2. The vapor enters heat transfer station 3 whereby a cooling medium enters as inlet 6 and exits at outlet 5 in countercurrent flow to entering vapor 8. As a consequence, the vapor condenses forming distillate 4.
Alternatively, distillation can be carried out in a continuous fashion as shown in FIG. 2. FIG. 2 differs from FIG. 1 only in the continuous feeding of liquid through inlet 9 into the still pot. Not surprisingly, the continuous process shown in FIG. 2 will result in an increase in concentration of component of the feed not converted to vapor and condensed as distillate. In this instance, when steady state operations have been achieved, the vapor composition remains constant. Both equipment designs of FIGS. 1 and 2 are effective only when the relative volatility among components is large.
Modern distillation equipment uses a rectifying column to achieve high mass transfer rates. Schematically, such a column is shown in FIG. 3 as element 20. The column contains perforated trays 21, 22 etc. or plates designed to allow the passage of liquid and vapor and produces large interfacial areas between vapor and liquid. As noted, liquid 24 is heated by reboiler 31 so that its vapor enters distillation column 20 at plate 21. These various trays or plates can incorporate devices called bubble caps to enhance contacting of liquid and vapor. Alternatives to trays include a large variety of proprietary shapes of packing materials such as Raschig rings, Lessing rings, Pall rings, Berl saddles and Tellerettes. As vapor from the feed liquid passes through the various plates or trays, it exits the top of the column through line 27 and enters a cooler or condensing area whereby a portion of the distillate can be removed through line 28 while a portion is recycled as liquid to pass through the column countercurrent to the vertically ascending vapor. Although FIG. 3 shows the feed entering the column at its lowest point, in operation, the feed can be introduced anywhere along the column depending upon various well-known parameters. As noted, condensed liquid can be caused to run down the column from tray to tray. Part of the liquid exiting the bottom of the column through line 26 can be introduced to the reboiler sending it back up the column. The remainder is withdrawn as bottom product through line 32. Make-up feed material enter the reboiler at 33.
In operation of the schematic device shown in FIG. 3, once thermal equilibrium has been reached, reflux liquid moving down the column comes into contact with rising vapor in a countercurrent fashion. This, in effect, provides multiple stages of vaporization and condensation. The composition of the distillate exiting line 27 corresponding with the temperature at the top of the column can be a very pure fraction of the vapor. Intermediate composition fractions may be tapped off intermediate points along the column length. All of the various operations are well known to those practicing unit operations in the field of chemical engineering.
The above description of the distillation process, while greatly simplified in the interests of brevity, is adequate to explain two of the problems associated with current equipment and explain the advantages to be obtained from the design approach to be described.
The first problem has to do with tunneling of liquid and vapor through the column structure. This and other forms of maldistribution limits full and effective utilization of the volume available producing a large spread in velocities and residence times for all components of the liquid vapor phases.
The second problem is associated with the degree to which liquid accumulates in the trays and associated contacting point sometimes referred to as "hang up." Clearly, if hang up is large, considerable time will have elapsed before equilibrium is reached between the two phases of liquid and vapor during which continuous operation is established. Pools of the liquid phase also reduce the contacting area between phases and this reduces mass transfer efficiency.
It is thus an object of the present invention to provide a novel distillation column which overcomes the problems inherent in the use of existing equipment design. This and further objects will be more readily apparent when considering the following disclosure and appended claims.